Photocurable composite materials are the most commonly used dental materials. Modern dental composites are usually a combination of an organic plastic matrix and inorganic fillers, wherein the inorganic phase and the organic phase are bonded together. The composition, which has not yet been cross-linked, thus contains free monomers or monomer mixtures, inorganic fillers or filler mixtures, initiators and in most cases colorants.
The curing of photocurable dental composite materials takes place by means of photoinitiated polymerization. As a result of this, following photochemical activation of an initiator system, chain reactions are started, wherein by means of three-dimensional cross-linking the liquid monomers are converted to a solid polymerizate.
The provision of prepared teeth with temporary prostheses, for example in crown and bridge prosthetics, is of particular importance to patients since these ensure the correct aesthetics, phonetics and occlusion. They also protect the hard dental material from caries and fractures and the pulp from trauma or injury. They also safeguard the dentine wound of the drilled tooth against thermal, chemical, mechanical and bacteriological stimuli, which within a very short time can lead to inflammatory processes in the pulp.
Various methods can be used in the production of temporary prostheses.
In the direct method, for example, prior to preparation with alginate or a silicon mass, a local impression is taken. Following preparation the plastic composition is introduced into the impression and placed again into the mouth of the patient.
Alternatively prefabricated plastic sleeves can also be used for example, wherein the temporary prosthetic is created without first taking an impression. The sleeves are individually tailored to the mouth by cutting and milling, filled with a self-polymerizing plastic and then placed on the prepared tooth.
Materials for the production of temporary crowns and bridges must guarantee the functioning of the prostheses for the time that they are present in the mouth. To this end the materials must meet a number of requirements.
Thus the materials must not release any toxic substances while in use. Harmful substances may be given off in the oral cavity through diffusion, friction, deterioration and by dental treatments such as grinding. Release of plastic monomers especially must be prevented as these are potential allergens and can leak into the oral cavity by diffusion in the form of unconverted residual monomers.
Furthermore, the material should have good mechanical properties, in particular in terms of hardness, strength, modulus of elasticity and abrasion behavior in order that, together with an optimum adaptation to the occlusal conditions, trouble-free functioning for the time in situ is ensured.
The surface of the temporary prosthesis should be smooth and able to be polished to a high gloss. The smoothness of the material serves to prevent the deposits of plaque while the high-gloss polishing capability seeks to meet aesthetic needs. A rough surface could also lead to irritation of the tongue and trigger paresthesia.
The temporary prosthesis, together with the securing material, should also exhibit a high level of tightness at its edges, in order to prevent the infiltration of bacteria and substances.
Furthermore, during the intra-oral phase of polymerization, the material should not cause any heat damage. The thermal energy released during cross-linking should not cause any pain in the mouth of the patient. At the same time the material should provide a complete and sealed covering of the ground tooth surface thereby protecting the tooth from thermal and mechanical irritation.
The crown and bridge materials used nowadays have a plastic base and are quite different in their chemical make-up. As a rule they contain methacrylates, acrylates and catalysts, inhibitors, cross-linking agents, plasticizers, accelerators, UV absorbers, fillers and so on. The curing of these products takes place either chemically through autopolymerization following the mixing of two pastes, the so-called base paste with the so-called catalyst paste in the static mixing tube of a double-chamber cartridge, or by dual-cured, i.e. both chemically and by means of irradiation at a suitable wavelength.
One distinct disadvantage of an autopolymerizable plastic composition is the heat developed during the exothermic polymerization which represents a danger to the pulp.
The quantity of heat released is dependent upon the dosing of the catalyst system used, the molding mass (impression substance) and the quantity processed. In the literature a number of working groups have measured differing temperature peaks for temporary crown and bridge materials which depending on the test set-up and the chemical composition vary between 25 and 94° C. Apart from the maximum temperature of the polymerization process the way in which the temperature develops is important. Thus for example the temperature development of the curing of a composition of a temporary crown and bridge material in a temperature-time diagram at the start of cross-linking appears relatively flat and then, around the point in time of the temperature maximum, briefly and suddenly increases and gives off a lot of heat within a very short space of time. On the other hand such a curve can also take a form in which a relatively long-lasting, but weaker, development of heat takes place.
Self-curing temporary crown and bridge materials are normally first applied in the impression mold. Here the usually two-component composition is pushed through a static mixing tube of a double chamber cartridge. During this process the components are mixed together and thus made to react. The impression mold filled with the mixed material is then inserted in the mouth and pushed on to the prepared stump. Once the radically curable plastic material in the impression mold has passed through its elastic phase it is removed from the stump after approximately 1 minute and taken out of the mouth where it finally cures extra-orally. Here the thermosetting also reaches its maximum temperature.
As early as during the elastic phase, where the maximum temperature has not yet been reached, a state of the dental composition is very quickly reached in which it starts to gel and the external shape essentially no longer changes. The viscosity of the composition is now so high that for the still un-reacted, unsaturated groups it becomes increasingly harder to obtain sufficient mobility to find a conversion partner. As a rule free monomers remain in the network and could subsequently be given off into the surroundings in the mouth.
A further disadvantage is the change in density in the material that occurs during cross-linking, which in the worst case scenario can lead to a change in the external form. If crown and bridge materials cure without keeping their shape, then this can later lead to a poor fit. The crown then no longer covers the stump completely. At these points microorganisms could later infiltrate and attack the tooth stump from the inside. This change in density is referred to as shrinkage or volume reduction of the reaction resin mass. This is primarily dependent upon the number of functional groups that have reacted. The shrinkage takes place both in the fluid state, thus at the very start of the polymerization, as well as during and after gelling. Overall shrinkage is subdivided into a physical and chemical component. Whereas physical shrinkage is directionally determined and runs spatially, from the outer areas of the polymerizate, operates in a similar fashion to and, in accordance with the drop in temperature that takes place, towards the central point of the molding material as it cures, the chemical component is not directionally determined, takes place solely as a result of the polymer formation and is highly dependent upon the geometrical conformation and configuration parameters of the newly constructed macromolecule.
With photocurable materials similar problems can arise. It has been found that a lower release of heat and lower shrinkage forces arise if the start of polymerization can be delayed from the outset. Such polymerization behavior can be achieved if, by having lower light power at the start and subsequently raising the light power to a maximum value, the polymerization is deliberately delayed. As a result of the lower light power at the start of curing the material remains flowable for longer, the gelling point is reached later, and the mobility of the functional groups is retained for longer so that higher conversion rates are achieved. The method used here is referred to as soft-start-polymerization. Together with the so-called “incremental method”, in which thin layers are individually photocured (very time-consuming), it is a further method that allows a reduction of high heat peaks and the shrinkage associated with the release of heat.
Approaches for providing low shrinkage dental materials, such as
the use of ring opening metathesis polymerization (ROMP) curing, byciclic monomers (DE 199 05 093 A1);
the addition of certain additives (cumarone resin, polyvinyl acetate, alcohol surfactants) prior to curing of the composition (DE 198 51 038 A1);
the use of cationic polymerizable oxetanes (U.S. Pat. No. 5,750,590);
the use of epoxy resins, containing nanoscale inorganic oxides and
the combination of different sized fillers (U.S. Pat. No. 6,709,271 B1)
have thus far not been able to establish themselves in practice.
For self-polymerizing and, to a much larger extent, for dual-curing dental compositions factors such as the release of heat and shrinkage represent problems that are currently countered primarily by the method and less so by the chemical formulation of the dental composition.
In EP 1 720 506, WO 2009/083168 A1, DE 10 2008 028 306 A1, EP 1 872 767 A1 and EP 2 070 506 various polymerizable dental materials are described. The dental materials disclosed there can be cured thermally, chemically, photochemically and/or though a reaction with the moisture in the mouth or air. Alongside many other stabilizers, these documents disclose, amongst others, in the respective lists of stabilizers terpinenes. A combination of photopolymerizable monomers and terpinenes is not disclosed in said documents.
In another area of application in dentistry, namely dental coatings, in systems for radical polymerization of photocurable masses the problem of high heat generation has in many cases been tackled either by adding non-reactive organic admixtures to the composition and/or using monomers with a high molecular weight, i.e. compounds with a low proportion of double bonds. In this connection U.S. Pat. No. 6,305,936 B1, U.S. Pat. No. 6,800,671 B1 and WO 2008/096182 are cited.
DE 42 33 886 C1 discloses polymerizable conditioning agents based on methacrylate and a method for pre-treating the surface of shaped bodies made from polyacrylate, polymethacrylate and polycarbonate plastics prior to application of polymerizable methacrylate material and use of the conditioning agent. The conditioning agent may contain monocyclic terpene hydrocarbon. Intra-oral use appears excluded due to the use of alkyl monomethacrylates, which should be categorized as posing a toxicological risk.
DE 27 27 480 A1 discloses impact-resistant, vitreous plastic alloys of polymethacrylates and aliphatic polyurethane ureas. Methods for the production thereof are disclosed, in which use is made of monomer mixtures comprising sulfur-containing molecular weight regulators. Not disclosed are compositions for the dental field.
DE 17 95 395 A discloses methods for producing polymers and copolymers of (meth)acrylic acid esters and acrylonitrile in the presence of polymerization catalysts and compounds having a six-membered ring containing two non-conjugated double bonds, one of which may be semicyclic (exocyclic). The specifically disclosed polymerization catalysts are thermal catalysts. Photoinitiators or photopolymerizable dental compositions are not disclosed.
U.S. Pat. No. 4,490,497 A discloses powder/liquid systems for a surgical cement, which can be used in the extra-oral production of dental prostheses. A liquid component of the composition comprises as the “chain stopper” for example a diunsaturated monocyclic terpene or a monounsaturated bicyclic terpene. Not disclosed are dual-curing compositions, in particular dual-curing dental compositions which are suitable for direct use in the oral cavity. Furthermore, the powder/liquid systems described are not suitable for distribution by means of dental syringes. The systems described in U.S. Pat. No. 4,490,497 A also have very long setting times. A link between the use of “chain stoppers” and polymerization stabilizers (such as alkyl-substituted monophenols) is not disclosed.
DE 30 10 373 A1 discloses methods for polymerizing methacrylic acid methyl esters or mixtures thereof with further vinyl monomers in the presence of enol ethers. The polymers produced by the disclosed method are referred to as being particularly suitable for use in the dental field, for example for producing prostheses according to the powder/liquid method; not disclosed, however, are dual-curing, multi-component compositions.
EP 1 720 506 B1 discloses a filled and polymerizable dental material and also a method for the production thereof. As use examples, the following are mentioned: tooth filling materials, stump build-up materials, materials for temporary crowns and bridges, dental cements, adhesives, materials for artificial teeth, veneering materials, sealing materials and dental varnish. It is disclosed that the dental material may contain additives and/or modifiers in order to set certain properties; a long list of examples includes “terpinenes”. Not disclosed, however, are dual-curing, multi-component compositions, which comprise a molecular weight regulator.
The person skilled in the art, e.g. the dentist, wants an extra-oral working time that is (as) long (as possible) and then an intra-oral setting time that is as short as possible after introduction into the mouth (i.e. generally at a slightly increased temperature). In this situation, the curing may not be accompanied by a large exotherm, since at high temperatures living tooth structure can suffer long-term damage.
In order to influence working time and setting time, a dental composition can be varied. For example, if in an otherwise identical composition the quantity added of a polymerization inhibitor in a radically polymerizable composition is increased, both the working time and the setting time increase steadily, wherein the delay in hardening is approximately proportional to the quantity added of polymerization inhibitor.
The object of the invention was to provide dental dual-curing (using dental chemistry with simultaneous photo-curing) compositions, which compared with the compositions of the prior art have a lower heat development and a lower shrinkage.
A specific dental task was to prepare a filled, dual-curing dental composition (in particular a composite material, which can be used as a dental filling material, as a flowable composite material, as a crown material, as a bridge material and/or as a stump build-up material), which should be able to be applied with a dental syringe and which should preferably be present in the form of two pastes, i.e. in the form of a paste/paste system.
Furthermore, the working time should be sufficiently long and at the same time the setting time should be as short as possible. Here, despite the shortest possible setting time, a temperature development that is as small as possible should take place, i.e. an acceptable temperature maximum during the curing of the dental composition in the oral cavity should not be exceeded.
The present invention concerns a dual-curing, multi-component dental composition comprising:
(a) one or more photopolymerizable monomers selected from the group consisting of acrylates and methacrylates, preferably from the group of methacrylates;
(b) one or more photoinitiators, selected from the group consisting of alpha-diketones, benzoin alkyl ethers, thioxanthones, benzophenones, acylphosphinoxides, acetophenones, ketals, titanocenes, borates and sensitizing colorants;
(c) one or more molecular weight regulators selected from the group consisting of compounds which can be converted with a radical of a monomer of component (a), wherein the conversion takes place by abstraction of an H-radical from the molecular weight regulator in the allyl position;(d) one or more polymerization inhibitors for increasing the storage stability of the composition, preferably selected from the group consisting of hydroquinone monomethyl ether (HQME), phenols, preferably 2,6-di-tert.-butyl-4-methylphenol (BHT) and tert.-butylhydroxyanisol (BHA), 2,2-diphenyl-1-picrylhydrazyl radicals, galvinoxyl radicals, triphenylmethyl radicals, 2,3,6,6-tetramethylpiperidinyl-1-oxyl radical (TEMPO) and derivatives thereof, and phenothiazine and derivatives thereof;(e) one or more inorganic fillers;(f) one or more initiators for a chemical curing at ambient temperature (preferably 23° C.), preferably a redox initiator system,and if necessary one or more further additives.
Surprisingly it has been found that dental compositions with a base of photopolymerizable monomers from the group of acrylates and methacrylates in the form of dual-curing, multi-component dental compositions according to the invention can be provided, which do not give off large quantities of heat into the surroundings, have low physical shrinkage and high cross-linking rates, and which following curing of the material no longer contain any harmful quantities of un-converted monomers which might subsequently have an adverse effect on the tissue.
Surprisingly it has also been found that a combination of one or more molecular weight regulators of component (c) and one or more polymerization inhibitors of component (d) in the dual-curing dental compositions according to the invention leads to such a curing behavior and such a polymerization kinetics relationship that after the mixing (e.g. in a static mixing tube) a long period of extra-oral working of the dental material is possible (cf. the remarks on the working time further below); however after intra-oral application, a rapid curing and setting takes place (cf. the remarks on the setting time further below).
Since the compositions according to the invention are dual-curing, the polymerization or the curing is normally accelerated further by exposure to light.
To summarize briefly, through the compositions according to the invention, in particular in one of the embodiments identified below as being preferred or particularly preferred, a comparatively long working time can thus be achieved within a certain context, without the setting time thereby also being extended.
The terms and definitions of “working time” and “setting time” and the methods for their determination relate within the context of the present text to DIN EN ISO 4049 (as at: March 2010).
The working times and setting times for different dual-curing, two-component dental compositions according to the invention containing 0.02 wt. % of the polymerization inhibitor BHT (2,6-di-tert.butyl-4-methylphenol; compound of component (d)) and different quantities of γ-terpinene (compound of component (c)) (in an otherwise identical composition) are illustrated in FIG. 1.
The working times and setting times for different dual-curing, two-component dental compositions not according to the invention containing different quantities of the polymerization inhibitor BHT (in an otherwise identical composition) in the absence of a molecular weight regulator according to component (c) are illustrated in FIG. 2.
According to the method of DIN EN ISO 4049, the working time is determined at a (forming) temperature of 23° C. and the setting time at 37° C. These (forming) temperatures correspond to normal extra-oral and intra-oral conditions.